It has been known in the prior art to drive a horn or other form of audible output transducer at a single frequency to indicate an alarm condition. In an indoor environment, single frequency driving of the horn or transducer can produce a complex set of zones of unacceptably low sound intensity. These are thought to arise from standing waves caused by the sound waves reflecting from surfaces, such as room surfaces, and a region being monitored.
The location of such low intensity zones is a function both of the frequency of the emitted sound from the transducer, or horn, as well as the locations of the reflective surfaces within the range of the sound. These zones will be substantially fixed in space for a specified frequency. The presence of unacceptably low sound intensity zones results in a circumstance where an alarm might not be clearly heard by a person who happened to be in any such zone.
FIG. 1 illustrates prior art drive circuitry 10 usable with sounders or other audible alarm emitting devices. Such devices would be suitable for use in fire monitoring systems.
Circuitry 10 includes a power source 12, which might be controllable, coupled to a single frequency wave form generator 14. The output of generator 14 can be amplified, amplifier 16, and then fed to an audible output transducer such as a piezoelectric horn 18. It will be understood that the circuit 10 is of a type which might be incorporated into a housing and pluralities of such devices might be distributed throughout a region being monitored. When activated, horn 18 emits outputs responsive to single frequency driving signals on line 16a. 
There continues to be a need for audible alarm indicating output devices which minimize or eliminate the presence of low sound intensity zones. Preferably, such improved devices would also project their output sound through doors and walls in the immediate area. In addition to improving sound penetration characteristics of such devices, it would also be desirable to be able to minimize power consumption on a per device basis since a given fire alarm system might incorporate a large number of such devices.